Recently, requirement for high-speed, high-effective working is becoming more serious in various industries, and the improvement of machine tools is remarkable. Along with the improvement of rotating speed of machine, edge temperature of a tool of machines elevates at cutting works and a sphere where ceramic tools are applied is broadly expanding, and a development of a ceramic tool is progressing. Especially, in the application of cutting tool such as drill, end mill, bite or throw away tips that the mounting/dismounting is freely made to various cutting tools, ceramic products are recognized as a promising material for tools applicable for high-speed cutting because of its excellent heat resistance, wear resistance and chemical stability, and is utilized for the manufacture of the tool for cutting work of cast iron. Recently, the ratio of a ceramic tool in the field of a cutting tool becomes about 10%.
However, a ceramic material is a brittle material and has weak points that chip and crack are easy to be generated. As an important feature to be required to a ceramic material tool, wear resistance and fracture resistance (fracture toughness) can be mentioned. When the wear resistance is bad, it is necessary to change a cutting tool frequently, and the productivity is deteriorated. Further, when the fracture resistance is bad, the sintered material is fractured at the cutting process and spoils the machined surface of the work. For the purpose to improve the cutting efficiency, it is necessary to prolong the life of a cutting edge so as to endure the long term use.
Accordingly, in a cutting tool composed of materials such as cemented carbide or ceramics, prolonging the life of a cutting edge is investigated by improving wear resistance or corrosion resistance of the cutting edge part by partially inserting a high pressure sintered materials such as cubic boron nitride or sintered diamond to the cutting edge, or treating by a specific heat treatment or coating by titanium carbide, titanium nitride, alumina or diamond like carbon.
In JPA H4-331070 publication (Document 1), invention relating a method to prolong the life of tool by making the surface structure of a cutting tool fine and minute that is composed of cemented carbide, SKH (high-speed tool steels) of hardening alloy steels, SKD (alloy tool steels) or carbon steels for machine structural use, SCM by blasting spherical abrasives of 300 mesh (approximately 50 μm) to 800 mesh (approximately 20 μm) particle size with air blow of 3-10 Kg/cm2 to the surface of said cutting tool is disclosed. The inventors of the present invention are already proposed in JPA 2002-300765 publication (filed on Oct. 15, 2002, Document 2), a method for surface toughening of a ceramic product comprising, forming uniformly distributed linear dislocation structure in the sub-surface regions of the ceramic product by using abrasives composed of fine particles having convexly curved surface and having an average particle size of 0.1 μm to 200 μm and a Vickers hardness (HV) of 500 or more and of a hardness (HV) of the ceramic products +50 or less, and a ceramic products whose surface is toughened. Further, in W. Pfeiffer and T. Frey, “Shot Peening of Ceramics: Damage or Benefit”, Ceramic forum international Cfi/Ber, DkG 79 No. 4, E25 (2002) (Document 3), toughening of ceramics by shot blasting is reported and a relationship between blasting materials, blasting pressure and toughening characteristic is investigated. However, method to adjust a shot blasting condition according to the relationship between formation of dislocation density of linear dislocation structure distributed uniformly measured by a transmission electron microscope and thermal shock resistance is not referred in this document.
Recently, the cutting conditions for cast iron or heat-resistance alloy are becoming more severe, and in a rough turning working or a milling cutter working, high-speed cutting over 1000 m/min is required. Further, in a case of high-speed cutting, the temperature of cutting edge becomes 1000° C. or more, (“Ceramics Engineering Handbook (2nd edition)” (Application) edited by Ceramic Society of Japan, Mar. 31, 2002 printed by GIHODO SHUPPAN Co., Ltd., p. 1285-1292, especially p. 1290, Document 4), and interrupted cutting processes are repeated in said condition, therefore an excellent thermal shock resistance is required. However, a conventional cutting tool characterized by inserting a high pressure sintered materials in a cutting edge part or by coating treatment with high hardness film has a problem that can not endure against such a severe high-speed cutting condition and causes fracture or delamination of coated film.
The subject of the present invention is to provide a method for the manufacture of tools whose heat resistance required in said cutting working is improved and whose life time is prolonged and to provide tools whose properties mentioned above are improved.
Aiming to accomplish said subject, the inventors of the present invention investigated whether the technique disclosed in the Document 2 by the inventors of the present invention can be applied for the improvement of said properties, and confirmed that fracture toughness, fracture resistance and thermal shock resistance can be improved and cutting tools whose life time is improved can be obtained, and accomplished above mentioned subject.